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2to3 is a Python program that reads Python 2.x source code and applies a series
of fixers to transform it into valid Python 3.x code. The standard library
contains a rich set of fixers that will handle almost all code. 2to3 supporting
library lib2to3 is, however, a flexible and generic library, so it is
possible to write your own fixers for 2to3. lib2to3 could also be
adapted to custom applications in which Python code needs to be edited
automatically.

A diff against the original source file is printed. 2to3 can also write the
needed modifications right back to the source file. (A backup of the original
file is made unless -n is also given.) Writing the changes back is
enabled with the -w flag:

Comments and exact indentation are preserved throughout the translation process.

By default, 2to3 runs a set of predefined fixers. The
-l flag lists all available fixers. An explicit set of fixers to run
can be given with -f. Likewise the -x explicitly disables a
fixer. The following example runs only the imports and has_key fixers:

$ 2to3 -f imports -f has_key example.py

This command runs every fixer except the apply fixer:

$ 2to3 -x apply example.py

Some fixers are explicit, meaning they aren’t run by default and must be
listed on the command line to be run. Here, in addition to the default fixers,
the idioms fixer is run:

$ 2to3 -f all -f idioms example.py

Notice how passing all enables all default fixers.

Sometimes 2to3 will find a place in your source code that needs to be changed,
but 2to3 cannot fix automatically. In this case, 2to3 will print a warning
beneath the diff for a file. You should address the warning in order to have
compliant 3.x code.

2to3 can also refactor doctests. To enable this mode, use the -d
flag. Note that only doctests will be refactored. This also doesn’t require
the module to be valid Python. For example, doctest like examples in a reST
document could also be refactored with this option.

The -v option enables output of more information on the translation
process.

Since some print statements can be parsed as function calls or statements, 2to3
cannot always read files containing the print function. When 2to3 detects the
presence of the from__future__importprint_function compiler directive, it
modifies its internal grammar to interpret print() as a function. This
change can also be enabled manually with the -p flag. Use
-p to run fixers on code that already has had its print statements
converted.

The -o or --output-dir option allows specification of an
alternate directory for processed output files to be written to. The
-n flag is required when using this as backup files do not make sense
when not overwriting the input files.

New in version 3.2.3: The -o option was added.

The -W or --write-unchanged-files flag tells 2to3 to always
write output files even if no changes were required to the file. This is most
useful with -o so that an entire Python source tree is copied with
translation from one directory to another.
This option implies the -w flag as it would not make sense otherwise.

Converts callable(x) to isinstance(x,collections.Callable), adding
an import to collections if needed. Note callable(x) has returned
in Python 3.2, so if you do not intend to support Python 3.1, you can disable
this fixer.

This optional fixer performs several transformations that make Python code
more idiomatic. Type comparisons like type(x)isSomeClass and
type(x)==SomeClass are converted to isinstance(x,SomeClass).
while1 becomes whileTrue. This fixer also tries to make use of
sorted() in appropriate places. For example, this block

Converts calls to various functions in the operator module to other,
but equivalent, function calls. When needed, the appropriate import
statements are added, e.g. importcollections. The following mapping
are made: